Electret electroacoustic transducers commonly comprise a conductive backplate and a permanently charged electret diaphragm that is separated from the backplate by a peripheral spacer. The surface of the electret diaphragm remote to the backplate is metalized, and the metalized surface is engaged by an electrically conductive member that maintains the diaphragm under tension. The electrical signal resulting from an acoustical signal impinging upon the diaphragm is applied to an impedance matching preamplifier circuit, and this circuit is commonly mounted adjacent to the backplate to facilitate electrically connecting the backplate to the circuit. Finally, to provide electrostatic shielding, these components are commonly assembled within an electrically conductive housing that makes electrical connection with the conductive tensioning member and provides a connection to a ground terminal.
While this basic structure is found in a variety of different arrangements, the problem has been that very few of these arrangements permit the use of automated manufacturing, assembly, and testing techniques.
One arrangement that appears to be directed toward this goal is disclosed in U.S. Pat. No. 3,775,572 issued to Ishibashi et al on Nov. 27, 1973. Ishibashi discloses a microphone which uses a group of three leads formed on a continuous strip. An integrated circuit chip is bonded adjacent to the upper end of one of the leads, and then wire connections are made between the circuit on the chip and the leads. This assembly is thereafter encapsulated in a disk-shaped insulating support with the leads extending parallel to the axis of and out the bottom surface of the support. The leads are then severed from the continuous strip, and one of the leads is cut off essentially flush with the bottom surface of the support. This same lead is of a height to extend close to the upper surface of the support, and the upper surface is lapped sufficiently to expose the end surface of this lead. A backplate is then either attached to the upper surface of the support or formed by evaporating metal on the upper surface, the backplate being thereby electrically connected to the lead by engagement with its exposed end surface. A ring-shaped insulating spacer and a diaphragm mounted to the underside of a ring-shaped conductive spacer are thereafter sequentially stacked on the support and the combination assembled within an inverted metal cup. The assembly is completed by stacking a disk-shaped insulating spacer and a conductive shield plate on the underside of the support, and then rolling over the lip of the metal cup against the shield plate to secure the assembly together.
This design was found by its corporate owner to be unsatisfactory in some respects. As stated in the introduction of U.S. Pat. No. 4,170,721, issued to Ishibashi et al, on Oct. 9, 1979, "If the conductive material used for the backplate is not coated on the insulating member uniformly, or if an upper surface of an insulating member is not formed flatly, the distance between the backplate and diaphragm is not uniform throughout." This subassembly must then be discarded.
The solution disclosed in this subsequent patent is a structure in which the backplate is encapsulated in a second insulating support, and the backplate is of a height to extend below the bottom surface of the second support. The lead that is to make electrical contact with the backplate, rather than being flush with the upper surface of the first support, extends above the upper surface, and an additional insulating member, which has an opening for accommodating the lower end of the backplate, is positioned between the first and second supports. Furthermore, a connector is interposed between the lead and the backplate to electrically connect one to the other. Since these components are in addition to the rest of the components of the first structure, it is seen that this solution adds significantly to the complexity of the structure.